Resume v2 polling on transient directory errors

[bc1cindy]

v2 poll loops resume on a transient (5xx) directory error instead of aborting the session, honoring RejectTransient "resume from current state" contract; fatal (4xx) still terminates.

stacked on #1658, do not merge first. The first 4 commits here belong to #1658 and drop out on rebase once it merges.

this PR's own 3 commits are viewable cleanly here: bc1cindy:transient-poll-retry

part of #1670

[coveralls]

Coverage Report for CI Build 27992226464

Coverage increased (+0.1%) to 85.34%

Details

• Coverage increased (+0.1%) from the base build.
• Patch coverage: 18 uncovered changes across 4 files (237 of 255 lines covered, 92.94%).
• 21 coverage regressions across 1 file.

Uncovered Changes

File	Changed	Covered	%
payjoin-cli/src/app/v2/mod.rs	61	49	80.33%
payjoin-mailroom/src/directory.rs	23	19	82.61%
payjoin-mailroom/src/db/mod.rs	22	21	95.45%
payjoin/src/core/schedule.rs	37	36	97.3%
Total (8 files)	255	237	92.94%

Coverage Regressions

21 previously-covered lines in 1 file lost coverage.

File	Lines Losing Coverage	Coverage
payjoin-mailroom/src/db/mod.rs	21	71.95%

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Coverage Stats

Relevant Lines:	14973
Covered Lines:	12778
Line Coverage:	85.34%
Coverage Strength:	380.65 hits per line

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💛 - Coveralls